Control system configured to compensate for non-ideal actuator-to-joint linkage characteristics in a medical robotic system

ABSTRACT

A medical robotic system having non-ideal actuator-to-joint linkage characteristics, includes a control system including a proximal control loop with actuator sensor feedback to control dynamic response of an actuator coupled to a distal joint which in turn, is coupled to an end effector to provide a degree of freedom movement of the end effector, a distal control loop with distal joint sensor feedback and feedforward to the actuator to ensure steady-state convergence of the distal joint position, and an end effector control loop with end-point sensor feedback to control the end effector position to reach a commanded end effector position.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/813,207 filed Jun. 13, 2006, which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention generally relates to medical robotic systems andin particular, to a control system configured to compensate fornon-ideal actuator-to-joint linkage characteristics in a medical roboticsystem.

BACKGROUND OF THE INVENTION

Medical robotic systems such as those used in performing minimallyinvasive surgical procedures offer many benefits over traditional opensurgery techniques, including less pain, shorter hospital stays, quickerreturn to normal activities, minimal scarring, reduced recovery time,and less injury to tissue. Consequently, demand for medical roboticsystems used for performing such minimally invasive surgical proceduresis strong and growing.

Examples of medical robotic systems include the daVinci® Surgical Systemand the daVinci® S™ Surgical System from Intuitive Surgical, Inc., ofSunnyvale, Calif. Each of these systems includes a surgeon's console, apatient-side cart, a high performance three-dimensional (“3-D”) visionsystem, and Intuitive Surgical's proprietary EndoWrist™ articulatingsurgical instruments or tools, which are modeled after the human wristso that when added to the motions of the robotic arm assembly holdingthe surgical instrument or tool, they allow at least a full six degreesof freedom of motion, which is comparable to or even greater than thenatural motions of open surgery.

In such a system, a patient-side cart typically has three or morerobotic arm assemblies each having a slave manipulator for holding andmanipulating a medical device such as a surgical tool or image capturingdevice for performing or viewing a medical procedure at a surgical sitewithin a patient. To manipulate these medical devices, the surgeon'sconsole also includes master manipulators which may be selectivelyassociated with the slave manipulators holding them. Typically, two suchmaster manipulators are provided, one for each hand of the operatingsurgeon.

Precise control in the positioning and manipulation of the surgicaltools and their end effectors is important for performing successfulmedical procedures on patients. Linkages coupling joint actuators todriven joints of the surgical tools or their manipulators are generallyfairly rigid so that surgical tool positions and velocities, and inparticular, those of their end effectors, may be reasonably and quicklyestimated in such medical robotic systems by applying kinematictransforms to sensed positions, velocities, accelerations, or torques ofthe actuators. However, when linkages are characterized bynon-idealities such as cable friction, hysteresis and compliance, toolpositions determined by applying kinematic transforms to sensedpositions, velocities, accelerations, or torques of the actuators mayresult in excessive estimation errors in the end effector positions andconsequently, diminished control capability for a surgeon performing amedical procedure.

Furthermore, the compliance and friction of the cable transmissionaffect the way the torques propagate from motors to the end effectors ofthe surgical tools. The capability of performing surgical procedures orgestures that require a fine control over the forces and torques appliedby the end effectors of the surgical tools on the manipulated tissues orobjects (e.g. suturing), is thus diminished.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, one object of one or more aspects of the present inventionis a control system with accurate position control of surgical toolsand/or their end effectors in the presence of non-negligiblenon-idealities in linkages coupling their associated actuators to theirdriven joints.

This and additional objects are accomplished by the various aspects ofthe present invention, wherein briefly stated, one aspect is a controlsystem for controlling an end effector of a medical device in a medicalrobotic system. Included in the control system are an end effectorcontroller configured to generate a first distal joint command using anend effector command; a first distal controller configured to generate afirst actuator command using the first distal joint command and a stateof a first distal joint; and a first proximal controller configured toclose a first feedback loop around a state of a first actuator to ensuretracking of the state of the first actuator, by generating a firstactuator feedback using the first actuator command and the state of thefirst actuator, and providing the first actuator feedback to the firstactuator so that the first actuator causes corresponding movement of thefirst distal joint and the end effector through a first transmissionelement.

Another aspect is a method for controlling movement of an end effectorof a medical device, comprising: generating a distal joint command usingan end effector command and a state of the end effector; generating anactuator command using the distal joint command and a state of a distaljoint mechanically coupled to the end effector so as to be capable ofmoving the end effector; generating an actuator feedback using theactuator command and a state of an actuator mechanically coupled to thedistal joint so as to be capable of moving the distal joint; andproviding the actuator feedback to the actuator so as to causecorresponding movements of the states of the actuator, the distal joint,and the end effector.

Additional objects, features and advantages of the various aspects ofthe present invention will become apparent from the followingdescription of its preferred embodiment, which description should betaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top view of an operating room employing a medicalrobotic system utilizing aspects of the present invention.

FIG. 2 illustrates a side view of a simplified robotic arm assemblyholding a surgical tool which may be manipulated by an input devicethrough a control system utilizing aspects of the present invention.

FIG. 3 illustrates a side view of a simplified robotic arm assemblyholding a single-port entry bundled unit of medical devices which may bemanipulated by input devices through control systems utilizing aspectsof the present invention.

FIG. 4 illustrates a perspective view of a distal end of a simplifiedsingle-port entry bundled unit of medical devices which may bemanipulated by input devices through control systems utilizing aspectsof the present invention.

FIG. 5 illustrates a block diagram of a control system for a medicalrobotic system, utilizing aspects of the present invention.

FIG. 6 illustrates a side view of a simplified surgical tool witharticulatable elbows suitable for insertion in a single-port entrybundled unit of medical devices for use with a medical robotic systemutilizing aspects of the present invention.

FIG. 7 illustrates a schematic diagram of a sensory architecture for amedical robotic system utilizing aspects of the present invention.

FIG. 8 illustrates a sensor classification diagram for a medical roboticsystem utilizing aspects of the present invention.

FIG. 9 illustrates a first embodiment of a linear displacement sensorfor use with a control system utilizing aspects of the presentinvention.

FIG. 10 illustrates a second embodiment of a linear displacement sensorfor use with a control system utilizing aspects of the presentinvention.

FIG. 11 illustrates a third embodiment of a linear displacement sensorfor use with a control system utilizing aspects of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates, as an example, a top view of an operating roomemploying a medical robotic system. The medical robotic system in thiscase is a minimally invasive robotic surgical system 100 utilized by aSurgeon 20 while performing a medical procedure, such as a diagnostic orsurgical procedure, with assistance from one or more Assistants 30, on aPatient 40 who is lying down on an operating table 50.

In this system, each medical device is shown to have its own associatedrobotic arm assembly for holding and manipulating the medical device(such as shown in FIG. 2), and its own associated incision for entryinto the Patient 40 for performing the medical procedure. In particular,each of the surgical tools 138, 139, as well as an image capturingdevice such as a stereoscopic endoscope 140, is conventionally insertedthrough a tool guide, trocar or cannula into the Patient 40 so as toextend down to the surgical site through its corresponding minimallyinvasive incision, such as incision 166 for the tool 138.

The number of medical devices used at one time and consequently, thenumber of robotic arms being used in the system 100 will generallydepend on the medical procedure being performed and the spaceconstraints within the operating room, among other factors. If it isnecessary to change a tool being used during a procedure, the Assistant30 may remove the tool no longer being used from its robotic armassembly and replace it with another tool 131 from a tray 60 in theoperating room.

When the medical robotic system includes a single-port entry device withsurgical tools and an image capturing device such as shown in FIG. 4,however, only one robotic arm assembly (such as shown in FIG. 3) andonly one port into the Patient 40 may be necessary to perform a medicalprocedure. The port in this case may be an incision such as incision 166or a natural body orifice through which the single-port entry device maybe robotically moved to a surgical site or workspace in the patient. Insuch a case, the control system and sensory architecture describedherein in reference to FIGS. 5-11 are also applicable, but with thesurgical tools 138, 139 and camera 140 understood as being held by onlyone robotic arm assembly such as robotic arm assembly 128, or otherappropriate driving mechanism.

The console 10 includes a 3-D monitor 104 for displaying a 3-D image ofa surgical site to the Surgeon, left and right manipulatable controldevices 108, 109 (also referred to herein as “master manipulators” or“input devices”), a foot pedal 105, and a processor 102. The controldevices 108, 109 may include any one or more of a variety of inputdevices such as joysticks, gloves, trigger-guns, hand-operatedcontrollers, or the like. The processor 102 may be a dedicated computerintegrated into the console 10 or positioned next or near to it, or itmay comprise a number of processing or controller components that aredistributed in a distributed processing fashion throughout the system100.

The console 10 is usually located in the same room as the Patient sothat the Surgeon may directly monitor the procedure, is physicallyavailable if necessary, and is able to speak to the Assistant(s)directly rather than over the telephone or other communication medium.However, it will be understood that the Surgeon can also be located in adifferent room, a completely different building, or other remotelocation from the Patient allowing for remote surgical procedures.

The Surgeon performs a medical procedure by manipulating the controldevices 108, 109 so that the processor 102 causes corresponding movementof their respectively associated surgical tools 138, 139, while theSurgeon views the surgical site in 3-D on the console monitor 104 as itis captured by the stereoscopic endoscope 140.

Preferably, control devices 108, 109 will be provided with at least thesame degrees of freedom as their associated tools 138, 139 to providethe Surgeon with telepresence, or the perception that the controldevices 108, 109 are integral with the tools 138, 139 so that theSurgeon has a strong sense of directly controlling the tools 138, 139.To this end, position, velocity, acceleration, force, and tactilefeedback sensors may be employed on the tools 138, 139 to transmitposition, velocity, acceleration, force, and tactile sensations from thetools 138, 139 back to the Surgeon's hands as he/she operates thecontrol devices 108, 109.

Preferably, the monitor 104 is positioned near the Surgeon's hands sothat it will display a projected image that is oriented so that theSurgeon feels that he or she is actually looking directly down onto theoperating site. To that end, images of the tools 138, 139 preferablyappear to be located substantially where the Surgeon's hands arelocated.

In addition, the real-time image is preferably projected into aperspective image such that the Surgeon can manipulate the end effectorsof the tools 138, 139 through their corresponding control devices 108,109 as if viewing the workspace in substantially true presence. By truepresence, it is meant that the presentation of an image is a trueperspective image simulating the viewpoint of an operator that isphysically manipulating the tools 138, 139. Thus, the processor 102transforms the coordinates of the tools 138, 139 to a perceived positionso that the perspective image is the image that one would see if theendoscope 140 was located directly behind the tools 138, 139.

The processor 102 performs various functions in the system 100. Oneimportant function that it performs is to translate and transfer themechanical motion of control devices 108, 109 to their respective slavemanipulator(s) through control signals over bus 110 so that the Surgeoncan effectively manipulate the tools 138, 139. Another importantfunction is to implement various control system and/or controllerprocesses as described herein.

Although described as a processor, it is to be appreciated that theprocessor 102 may be implemented in practice by any combination ofhardware, software and firmware. Also, its functions as described hereinmay be performed by one unit or divided up among different components,each of which may be implemented in turn by any combination of hardware,software and firmware. Further, although being shown as part of or beingphysically adjacent to the console 10, the processor 102 may alsocomprise a number of subunits distributed throughout the system such asin printed circuit boards installed in the patient side cart 120 andslave manipulators of the robotic arm assemblies 128, 129, 122, as wellas, or alternatively to, the console 10.

For additional details on the construction and operation of variousaspects of a medical robotic system such as described herein, see, e.g.,commonly owned U.S. Pat. No. 6,493,608 “Aspects of a Control System of aMinimally Invasive Surgical Apparatus,” and commonly owned U.S. Pat. No.6,671,581 “Camera Referenced Control in a Minimally Invasive SurgicalApparatus,” which are incorporated herein by reference.

FIG. 2 illustrates, as an example, a side view of a simplified (notnecessarily in proportion or complete) robotic arm assembly 200 (whichis representative of the robotic arm assemblies 128, 129) holding asurgical tool 250 (which is representative of tools 138, 139) forperforming a medical procedure. A tool guide 270 is inserted through aminimally invasive incision in a patient, and coupled to the robotic armassembly 200 by a guide holder 240. The surgical tool 250 may then beinserted into the patient through the tool guide 270. The robotic armassembly 200 is mechanically supported by a base 201, which may be partof a patient-side movable cart or affixed to the operating table orceiling.

Links 202, 203 are coupled together and to the base 201 throughhorizontal setup joints 204, 205. The setup joints 204, 205 in thisexample are passive joints that allow manual positioning of the arm 200when their brakes are released. For example, setup joint 204 allows link202 to be manually rotated about axis 206, and setup joint 205 allowslink 203 to be manually rotated about axis 207.

Although only two links and two setup joints are shown in this example,more or less of each may be used as appropriate in this and otherrobotic arm assemblies in conjunction with the present invention. Forexample, although setup joints 204, 205 are useful for horizontalpositioning of the arm 200, additional setup joints may be included anduseful for limited vertical and angular positioning of the arm 200. Formajor vertical positioning of the arm 200, however, the arm 200 may alsobe slidably moved along the vertical axis of the base 201 and locked inposition.

The robotic arm assembly 200 also includes two active joints and anumber of gears driven by motors. A yaw joint 210 allows arm section 230to rotate around an axis 261, and a pitch joint 220 allows arm section230 to rotate about an axis perpendicular to that of axis 261 andorthogonal to the plane of the drawing.

The arm section 230 is configured so that sections 231, 232 are alwaysparallel to each other as the pitch joint 220 is rotated by its motor.As a consequence, the instrument 250 may be controllably moved bydriving the yaw and pitch motors so as to pivot about the pivot point262, which is generally located through manual positioning of the setupjoints 204, 205 so as to be at the point of entry into the patient. Inaddition, the surgical tool 250 is coupled to a carriage 245 on the armsection 230 which in turn is coupled to a linear drive mechanism toextend or retract the tool 250 along its insertion axis 263. Aninterface 252, which includes mating parts of the motor driven gears onthe carriage 245 and a proximal end of the tool 250, facilitates drivingwrist joints and other drivable members of an end effector 251 of thetool 250 using conventional gear, pulley and cable arrangements, as wellas facilitating rotation of a shaft of the tool 250 so as to provideroll angular movement of the tool 250 about its insertion axis 263.

Although each of the yaw joint 210, pitch joint 220 and motor drivengears in the carriage 245 is controlled by an individual joint or gearcontroller, the controllers may be controlled by a common master/slavecontrol system such as shown in FIG. 5, so that the tool 250 may becontrolled through user (e.g., surgeon or operator) manipulation of itsassociated control device.

FIG. 3 illustrates, as an example, a simplified side view of the roboticarm assembly 200 adapted this time to hold and manipulate a single-portentry device 300, and FIG. 4 illustrates, as an example, a perspectiveview of a distal end of the single-port entry device 300. Thesingle-port entry device 300 includes removable surgical tools 338, 339for performing a medical procedure and a removable image capturing unit340 for viewing the procedure at a surgical site within a patient. Eachof the tools 338, 339 and image capturing unit 340 extends through aseparate lumen formed in an inner core of the single-port device 300.Replacement of one or both of the surgical tools 338, 339 during or inpreparation for performing a medical procedure may then be accomplishedby removing the tool that is no longer needed from its lumen andreplacing it with a substitute tool by inserting the substitute tool inthe vacated lumen. Alternatively, if unused lumens are available, anadditional tool may be inserted through one of those available lumenswithout removing any other tools already in place.

The image capturing unit 340 preferably includes a stereoscopic pair ofcameras 342, 343 for three-dimensional imaging of the surgical site andan illuminating device 344 such as a light emitting diode (LED) or afiber optics bundle carrying light from an external source, to enhancevisibility of objects in the captured images. Auxiliary image capturingunits, such as an ultrasound probe, may also be provided in availablelumens of the single-port entry device 300 for “seeing” into anatomicstructures for surgical or diagnostic purposes.

In some embodiments, an overtube 310 is also included in the single-portdevice 300 for protecting its inner core and the medical devices (i.e.,surgical tools and image capturing units) inserted therethrough. Theovertube 310 may be rigid. Alternatively, it may be formed of flexiblematerial or comprise actively and/or passively bendable sections so thatthe single-port device 300 may conform to the shapes of body lumens asit moves therethrough to a surgical site within a patient.

The single-port device 300 is inserted into the tool guide 270 which iscoupled to the guide holder 240 of the robotic arm assembly 200. Aspreviously described in reference to FIG. 2, movement of the robotic armassembly 200 causes the tool guide 270 and the single-port device 300inserted therein, to pivot about the pivot point 262 which is preferablythe point of insertion of the tool guide 270 into the patient body. Theinsertion point may be a minimally invasive surgical incision or anatural body orifice connected to a body lumen leading to a surgicalsite within the patient.

An interface 302 comprising mating parts on the carriage 245 and theproximal end of the single-port device 300 such as motor driven gearsthat actuate movement of the surgical tools 338, 339 and image capturingunit 340 through conventional joints, cable and pulley systems. Thesurgical tools 338, 339 are equipped with end effectors 322, 332 thatare configured with wrist mechanisms 323, 337 that allow the endeffectors 322, 332 to be oriented in various poses. The surgical tools338, 339 are also equipped with controllably extendable and bendablearms that facilitate positional and orientational movement of the endeffectors 322, 332. To facilitate their controlled bending, bend sensorsmay be employed in the arms. Joint sensors may also be employed fordetermining end effector positions based upon kinematic calculations.

As shown in FIG. 4, the surgical tools 338, 339 each have a controllablyextendable, rotatable, and bendable arm to which their respective endeffectors 322, 332 are coupled to by wrist mechanisms 323, 337. Forexample, the arm of the surgical tool 339 comprises three links 331,333, 335 coupled by distal joints 334, 336. The proximal link 335 iscontrollably extendable and retractable along an insertion axis 352(which is preferably parallel to the insertion axis 263 of thesingle-port device 300), and is controllably rotatable (as shown byrotation angle 353) about the insertion axis 352. The middle link 333,on the other hand, is controllably bendable by distal joint 336 relativeto the link 335 (as shown by bend angle 351), and the distal link 331 iscoupled to the links 333, 335 and bendable by distal joint 334 so thatits bend angle 354 is in an opposite direction as that of the link 333and consequently, keeps links 331, 335 in parallel alignment.

The arm of the surgical tool 338 is similarly constructed as that of thesurgical tool 339. Additional details for one example of the wristmechanisms 323, 337 are provided in commonly owned U.S. Pat. No.6,817,974 “Surgical Tool Having Positively Positionable Tendon-ActuatedMulti-Disk Wrist Joint,” which is incorporated herein by this reference.

The image capturing unit 340 also has a controllably extendable,rotatable, and bendable arm 345 that facilitates at leastinsertion/retraction of the image capturing unit 340 along its insertionaxis (which may be parallel to the insertion axis 263 of the single-portdevice 300) and pitch motion in order to achieve a sufficient elevationof the image capturing device 340 “above” the surgical tools 338, 339 soas to properly view them during a surgical procedure. Additional degreesof freedom, such as roll angular movement of the image capturing device340 about its insertion axis, may also be provided in order tofacilitate additional positioning and orientation capabilities for theimage capturing device 340. For enhanced maneuverability, the imagecapturing arm 345 may also be bendable such as the controllablybendable, rotatable, and extendable arms of the surgical tools 338, 339.

FIG. 5 illustrates, as an example, a block diagram of a control system500 used for controlling the end effector 332 of the surgical tool 339,and FIG. 6 illustrates, as an example, a side view of a simplifieddepiction of the surgical tool 339. Although certain types of sensorsare described as being used in the following description of the controlsystem 500, it is to be appreciated that the described sensors shall notbe limiting. In particular, various types of sensors may be used and arefully contemplated to be within the scope of the present invention, suchas those shown and described in reference to FIGS. 7 and 8. Accordingly,it is to be understood that any type of sensor (or combinations thereof)that senses any aspect of the state of a controlled element over time,such as its position (either linear or angular), velocity, acceleration,orientation, and/or forces and torques exerted by it or against it maybe used in alternative embodiments of the control system 500.

Note that the surgical tool 339 depicted in FIG. 6 is preferablyutilized in minimally invasive surgical devices having several distalarticulations that are characterized by non-negligible compliance andhysteresis characteristics in cable transmissions actuating the toolmechanism. For example, the tool 339 may used in a single-port entrydevice since the additional distal degrees of freedom provided by itsarticulatable elbows (e.g., distal joints 334, 336 for the tool 339)serve to help compensate for the lack of independent robotic armassemblies for each of the surgical tools 338, 339. As another example,it may be used as, or as a part of, a flexible articulated device (e.g.with an actively controlled flexible entry guide) whose shape iscontrolled in order to minimize its contact with soft tissue of thePatient, as well as manipulation of its end effector 332 for performinga medical procedure on the Patient.

Referring first to FIG. 6, the surgical tool 339 includes three basiccomponents: an actuator 523, the distal joint 334 and the end effector332. The actuator 523 is preferably a motor which has a rotor driving acable (or system of cables) connected to the distal joint 334 so as tocontrollably bend the distal joint 334. It is able to do this, becausethe distal joint 334 is coupled to the distal joint 336 (which in turn,is coupled to the actuator 523) so as to move with it, but in anopposite rotation as shown in FIG. 4, using a multiple cable scheme asdepicted therein. As an example, a primary or actuation cable 600 iswrapped around a rotor of the actuator 523. The ends of the primarycable 600 extend through a hollow of the distal arm link 335, and aretied to opposing points 601, 602 on an inner wall of a distal end of themiddle arm link 333. When the rotator rotates, it causes the primarycable 600 to be pulled in one direction or another (depending upon thedirection of the rotation) and the distal joint 336 to bend in thedirection of the pull. Secondary or constraint cables 610, 620, on theother hand, are not directly connected to any actuator, but extendthrough a hollow of the middle arm link 333, and have distal ends tiedto opposing points 611, 621 of an inner wall of the distal arm link 335and proximal ends tied to opposing points 612, 622 of an inner wall ofthe proximal arm link 331, so that when the actuator 523 causes thedistal joint 336 to bend in one direction (by causing the primary cable600 to be pulled in that direction), the secondary cables 610, 620 causethe distal joint 334 to bend in a substantially equal, but oppositedirection.

The distal joint 334, in turn, is connected to the end effector 332 sothat bending of the distal joint 334 results in movement of the endeffector 332. In this example, an encoder 524 is coupled to the actuator523 to sense the rotation angle φ₁ of its rotor, a distal joint sensor526 is positioned proximate to or within the distal joint 334 to senseits bend angle θ₁, and an end-point sensor 551 is mounted on orproximate to the end effector 332 so as to sense its position in areference frame 610. Examples of the encoder 524, distal joint sensor526, and end-point sensor 551 are described below in reference to FIG.8. Note that the encoder 524 can be replaced by any other type of localsensor that is appropriate to close a local feedback loop to control theactuator 523 in order to achieve its desired state.

Referring now to FIG. 5, the control of two distal joints 334, 544 isdepicted therein, each joint controlling one degree of freedom movementof the end effector 332. As shown in FIG. 4, the distal joint 336controls bending of a middle arm link 333 of the end effector 332relative to the proximal arm link 335. The distal joint 544, on theother hand, may control rotation of the proximal arm link 335 about theinsertion axis 352. Additional distal joints and corresponding jointcontrol loops may also be added to the control system 500, such as, forexample, controlling the insertion/retraction of the proximal arm link335 along the insertion axis 352, or controlling any of the wrist jointsof the wrist mechanism 337.

A Cartesian end effector controller 510 receives an end effector commandX_(ref) and information generated by an end-point detector 551 of aposition “X” of the end effector 332. The Cartesian end effectorcontroller 510 generates, through inverse kinematics computations, firstand second distal joint commands θ_(1ref), θ_(2ref) using the twoinputs. The end effector command X_(ref) is received from one of thecontrol devices 108, 109 associated with the end effector 332 andtypically represents an operator manipulated displacement of the controldevice in its frame of reference. The end-point sensor 551 is typicallymounted on, or otherwise proximate to, the end effector 332, so as toprovide information from which its position X, and eventually itsorientation, may be determined.

The Cartesian end effector controller 510 translates the end effectorcommand X_(ref) into the reference frame of the end effector position Xand computes an end effector position error X_(ERR) as a differencebetween the translated end effector command X_(ref) and the end effectorposition X. Alternatively, both the end effector command X_(ref) and theend effector position X may be translated to a common reference framesuch as a camera reference frame, a surgeon eye reference frame, or aworld reference frame that remains fixed throughout the medicalprocedure. The end effector position error X_(ERR) is then translatedinto first and second distal joint commands θ_(1ref), θ_(2ref), usingconventional transform techniques.

A distal controller 521 receives the distal joint command θ_(1ref) fromthe Cartesian end effector controller 510 and information of a distaljoint bend angle θ₁ sensed by the distal joint sensor 526, computes adistal joint position error θ_(1ERR) as a difference between the two,and translates the error into an actuator command φ_(1ref). In a similarmanner, a proximal controller 522 receives the actuator command φ_(1ref)from the distal controller 521 and information of an actuator positionφ₁ sensed by the encoder 524, computes an actuator position errorφ_(1ERR) as a difference between the two, and translates the error intoan actuator feedback error φ_(1E) which is provided through a summingnode 525 to the actuator 523.

Due to the compliance, friction, backlash and hysteresis experienced bythe cable (or other linkage) that mechanically couples the actuator 523to the distal joint 334, the Cartesian end effector controller 510preferably also computes a feed-forward torque command τ_(1FF) tocompensate for such transmission non-idealities and provides it to thesumming node 525 so that it is summed with the actuator feedback errorcommand φ_(1E) to generate a compensated actuator command φ_(1C). Thecompensated actuator command φ_(1C) is then provided to the actuator 523to ensure that a target steady-state condition for the distal joint bendangle θ₁ converges to its commanded position θ_(1ref) during the controlprocess. The feed-forward torque command τ_(1FF) in this case iscalculated using appropriate friction and compliance models for thecable so as to compensate for its non-idealities. Without thefeed-forward torque command τ_(1FF), the distal joint bend angle θ₁ mayhave difficulty converging to its target steady-state commanded value,even though the actuator position error φ_(1ERR) is zeroed by the innercontrol loop controlled by the proximal controller 522.

Movement of the distal joint 334 thereupon causes corresponding movementof the end effector 332 through a mechanism 540 coupling the two.Control of the second distal joint 544 (and any other distal jointscontrolling other degrees of freedom movement of the end effector 332)is performed in the same manner as described in reference to the controlof the first distal joint 334 above.

A key feature of the control system 500 is its use of multiple controlloops. In an inner loop, the proximal controller 522 zeroes the actuatorposition error φ_(1ERR). This inner loop is a relatively high-speed loopthat tightly controls the dynamic performance of the control system 500.In a middle loop, the distal controller 521 zeroes the distal jointposition error φ_(1ERR). This middle loop is generally slower performingthan the inner loop, because the distal joint sensor 526 is generallyslower and has lower resolution than the encoder 524. However, becauseof the relative slow motion of the Surgeon's hands (typically in therange of 0 to 15 Hz) compared to the bandwidth of the inner controlloop, the distal joint sensor 526 is capable of providing additionalcompensation for non-ideal actuator-to-joint linkage characteristics inthe range of frequencies that are important to surgical applications.Addition of the feed-forward torque command τ_(1FF) allows the distalcontroller 521 to ensure steady-state convergence to its target valuefor the distal joint angle θ₁ and consequently the end effector positionX. Finally, in the outermost loop, the end effector controller 510zeroes the end effector position error X_(ERR) and in so doing providesmore refined compensation of the cable non-idealities.

The control system 500 may be used in applications where it is desirableto achiever besides fine position control, a refined control of theinteraction forces and torques between the end effector 332 of thesurgical tool 339 and the patient tissues or manipulated objects (e.g.when suturing). Indeed the combination of local sensing at the actuator523, distal joint position sensing at the distal joint 334, and endpoint sensing at the end effector 332 provides information of theoverall hysteresis and compliance of the cable transmission from whichthe Cartesian end effector controller 510 is able to estimate the cabletension and therefore to adjust the actuator command φ_(1ref) in such away that the interaction forces and torques at the end effector can becontrolled with better accuracy than in the case of a simple local loopclosed around the actuator 523.

Also, in this example, the direct knowledge of distal joint angles θ₁,θ₂ allows the Cartesian end effector controller 510 to accuratelycompute the Jacobian matrix for the direct and inverse kinematictransforms used to convert the end effector position error X_(ERR) tofirst and second distal joint commands θ_(1ref), θ_(1ref). In highlyminiaturized surgical devices, however, direct joint positionmeasurements may not be available because of size constraints. In thatcase, only end-point sensing may be available and the joint angles θ₁,θ₂ may not be known with accuracy because of non-idealities in the cabletransmission. Thus, in this case, inaccuracies may occur in thekinematic transformations from the Cartesian set-point X_(ref) to theend effector position X due to the absence of reliable positioninformation for the joints. In such case, joint models may be employedalong with actuator position and torque sensors to estimate the distaljoint positions θ₁, θ₂ for use in the control system 500 as substitutesfor the distal joint sensors 526, 536. To improve the accuracy of themodel, the model may be updated using information sensed by a jointcontact, proximity, force, or other type of sensor that may be too slowfor direct feedback purposes (unlike the distal joint sensors 526, 536),but accurate enough for updating the joint models. Note, however, thatif the end effector 332 is attached to fully actuated, non-redundant,six degrees-of-freedom (“6 DOF”) slave manipulator, a 6 DOF sensor onthe tip of the end effector 332 may provide sufficient information touniquely determine the distal joint angles by means of inversekinematics transforms. In this case, the tip sensor information,combined with the encoder measurements and forward kinematicstransforms, directly relates to the current amount of “stretch” or“slop” in the cable transmission and may be used to generate refinedactuator commands.

FIG. 7 illustrates, as an example, a schematic diagram of a sensoryarchitecture for constructing a map of the Patient's anatomy andproviding navigational aids to help the Surgeon navigate the single-portentry device 300 (or other medical device) along complex body structuresto a surgical site within the Patient using the medical robotic system100.

Besides information provided by proprioceptive sensors such as thoseused in the control system 500 for position control, informationprovided by exteroceptive sensors mounted on or in adjacent lumens tothe surgical tools 338, 339, such as force and ultrasound sensors, alongwith information provided by external sensors, can be utilized to locatethe single-port entry device 300 in an absolute reference frame and withrespect to the Patient's anatomy. Registration with pre-operativeimaging can also be used to compute virtual fixtures as navigationalaids that allow the Surgeon to avoid contact with delicate organs orblood vessels of the Patient.

The relative position of the surgical tools 338, 339 with respect to thesurrounding environment is used to further refine their respective endeffector position and distal joint bending control. Together,map-building, self-localization, and exteroceptive sensors may be usedto provide haptic and visual aids to the Surgeon in order to helpnavigate through the Patient's complex anatomical structures.

Sensor fusion techniques may be used to combine information fromdifferent sources for control, localization, map building, andnavigational purposes. Since each sensor is characterized by its ownnoise levels, accuracy, and bandwidth, it is likely that at any giventime a certain level of contradiction or inconsistency exists betweeninformation being provided by different sensors. Probabilisticalgorithms may be used to overcome this problem and to reduce the impactof outliers on the map-building and localization processes involved withconventional Simultaneous Localization and Map Building (SLAM)techniques. In particular, Bayesian techniques and Extended KalmanFilters (EKE) provide the theoretical foundations to the successfulimplementation of the scheme depicted in FIG. 7. As an example of usingsuch Bayesian techniques, see commonly owned U.S. patent applicationSer. No. 11/130,471 entitled “Method and System for Performing ToolTracking during Minimally Invasive Robotic Surgery,” filed May 16, 2005,which is incorporated herein by this reference.

A significant advantage of using sensor fusion in a global datastructure such as shown in FIG. 9 is that the sensory architecture isintrinsically modular. For example, the Surgeon might use an ultrasonicsensor at one point of a medical procedure and replace it at a latertime during the procedure with a force sensor in order to determine amore accurate and complete model of the tissue of interest. To enablethis type of flexibility, a specific sensor model may be associated toeach sensorized instrument. Once such models are available, thesensor-fusion architecture allows the number and type of sensors used tobe changed during a medical procedure (i.e., inter-operatively) whileretaining a consistent and unified sensory architecture for control,self-localization, map building, and navigational purposes.

FIG. 8 illustrates, as an example, a sensor classification diagram forthe medical robotic system 100. On board sensors include proprioceptiveand exteroceptive sensors that are includable in the robotic medicalsystem 100, whereas external sensors such as Magnetic Resonance Imaging(MRI) scanners, fluoroscopes, magnetic and optical landmark detectors,and external trackers are generally not included as part of the roboticmedical system 100 but provide additional information for the surgeon.

The proprioceptive sensors are primarily used in the control system 500.In particular, proximal sensors are primarily usable as the encoders524, 534 in the control system 500, such as motor encoders and torquesensors for the actuators 523, 533. Distal end-point sensors areprimarily usable as the end-point sensor 551 in the control system 500,such as end-point mounted inertial sensors, accelerometers, andelectromagnetic trackers. Micro Electro Mechanical System (MEMS) gyrosand accelerometers, for exampler are particularly useful for suchapplication.

Distal individual joint sensors are primarily usable as the distal jointsensors 526, 536 in the control system 500, such as distributed shapesensors, linear displacement sensors, and reference element sensors. AFiber-Bragg Grating sensor is one example of a distributed shape sensor,for which additional details are provided in commonly owned U.S. patentapplication Ser. No. 11/491,384 entitled “Robotic Surgery Systemincluding Position Sensors using Fiber Bragg Gratings,” filed 20 Jul.2006, which is incorporated herein by this reference.

Some examples of linear displacement sensors and reference elementsensors are illustrated in FIGS. 9-11. In each of these examples, acable 901 mechanically couples the actuator 523 to the distal joint 334in a conventional manner so that a displacement φ₁ of the actuator 523causes linear movement of the cable 901 (such as in the direction “D”)and a corresponding displacement θ₁ of the distal joint 334. It is knownthat in such arrangements, friction and hysteresis of the cable 901between the actuator 523 and the start of the cable-driven distal joint334, due to the cable 901 going around bends and extending for longlengths, can introduce substantial uncertainties in the fine control ofthe articulated distal joint 334. Therefore, the inclusion of the distaljoint sensor 526 in the control system 500 can provide substantialimprovement over prior art control systems lacking such a sensor.

In FIG. 9, linear movement of the cable 901 is sensed by a lineardisplacement sensor 902 at a reference point P1 on the cable 901. Thereference point P1 is selected in this case so as to be sufficientlynear the distal joint 334 that movement of the cable 901 at the point P1is a good indication of the movement of the distal joint 334. Examplesof linear displacement sensors usable in this case include a LinearVoltage Differential Transformer (LVDT), a linear Hall effect sensor anda little magnet, and a slide potentiometer.

Another example of a linear displacement sensor is the sensor 1002depicted in FIG. 10, where linear displacement of the cable 901 isestimated using the sensor 1002 by measuring the resistance Ω of arubber part 1001 having one end attached to the cable 901 at thereference point P1 and another end attached to a fixed point P0. Stillanother example of measuring linear displacement is depicted in FIG. 11,where an unloaded reference cable 1101 is attached to cable 901 at thereference point P1 and its linear displacement sensed by a lineardisplacement sensor 1102, which may be the same type of lineardisplacement sensor as described in reference to FIG. 9.

Referring back to FIG. 8, exteroceptive sensors may also be used tofacilitate or enhance Surgeon control of the surgical tools 338, 339 andtheir end effectors 322, 332, or aid the Surgeon in performing a medicalprocedure by providing auxiliary information of the target anatomy orsurgical site. Examples of these sensors include ultrasound probes,chemical and temperature sensors, force and pressure sensors, and visionsystems that perform image processing functions such as edge detection,identification of markers on the instruments, and optical flow. Visionsystems with image processing may further be used as primary orsupplemental end-point and/or distal bend sensors, provided the endeffectors and/or distal joints are within the field of view of theircameras and the positions of their cameras are accurately known.

Although the various aspects of the present invention have beendescribed with respect to a preferred embodiment, it will be understoodthat the invention is entitled to full protection within the full scopeof the appended claims.

1. In a medical robotic system, a control system for controlling an endeffector of a medical device, the control system comprising: an endeffector controller configured to generate a first distal joint commandusing an end effector command indicative of operator manipulation of aninput device; a first distal controller configured to generate a firstactuator command using the first distal joint command and a state of afirst distal joint coupled to the end effector for effecting a firstdegree of freedom movement of the end effector; and a first proximalcontroller configured to close a first feedback loop around a state of afirst actuator to ensure tracking of the state of the first actuator, bygenerating a first actuator feedback using the first actuator commandand the state of the first actuator and providing the first actuatorfeedback to the first actuator so that the first actuator causescorresponding movement of the first distal joint and the end effectorthrough a first transmission element.
 2. The control system according toclaim 1, wherein the first transmission element is characterized bynon-idealities.
 3. The control system according to claim 1, wherein theend effector controller is configured to generate the first distal jointcommand using the end effector command and a state of the end effector.4. The control system according to claim 3, further comprising anend-point sensor adapted to generate information from which the state ofthe end effector is determinable.
 5. The control system according toclaim 4, wherein the end-point sensor comprises an inertial sensor. 6.The control system according to claim 5, wherein the inertial sensorcomprises a gyroscope.
 7. The control system according to claim 6,wherein the inertial sensor comprises a micro electro mechanical systemdevice.
 8. The control system according to claim 4, wherein theend-point sensor comprises an accelerometer.
 9. The control systemaccording to claim 8, wherein the accelerometer comprises a microelectro mechanical system device.
 10. The control system according toclaim 4, wherein the end-point sensor employs an electromagnetictracker.
 11. The control system according to claim 3, wherein the endeffector controller is configured to generate a feed forward command tocompensate for residual error in the first actuator feedback due tonon-ideal characteristics of the transmission element, and provide thefeed forward command so as to be summed with the first actuator feedbackand provided to the first actuator.
 12. The control system according toclaim 11, wherein the proximal controller is configured to substantiallycontrol dynamic performance of the control system, and the distalcontroller is configured to substantially control steady-stateperformance of the control system.
 13. The control system according toclaim 1, further comprising a first distal joint sensor adapted togenerate information from which the state of the first distal joint isdeterminable.
 14. The control system according to claim 13, wherein thefirst distal joint sensor comprises a distributed shape sensor.
 15. Thecontrol system according to claim 13, wherein the first distal jointsensor comprises a linear displacement sensor.
 16. The control systemaccording to claim 15, wherein the linear displacement sensor is adaptedto provide information from which a distance that the first transmissionelement moves is determinable.
 17. The control system according to claim16, wherein the linear displacement sensor comprises a linear voltagedifferential transformer.
 18. The control system according to claim 16,wherein the linear displacement sensor employs a Hall effect sensor anda magnet.
 19. The control system according to claim 16, wherein thelinear displacement sensor comprises a slide potentiometer.
 20. Thecontrol system according to claim 16, wherein the linear displacementsensor employs a reference element coupled to the transmission elementat a reference point so as to measure movement of the reference elementas an indication of movement of the transmission element.
 21. Thecontrol system according to claim 1, wherein the medical devicecomprises a single-port entry device having a plurality of surgicaltools controllably extendable out of a distal end of the single-portentry device.
 22. The control system according to claim 21, wherein thefirst transmission element comprises a first cable, and movement of thefirst cable is indicative of movement of the first distal joint.
 23. Thecontrol system according to claim 1, wherein the end effector controlleris configured to generate a second distal joint command using the endeffector command, and further comprising: a second distal controllerconfigured to generate a second actuator command using the second distaljoint command and a state of a second distal joint coupled to the endeffector for effecting a second degree of freedom movement of the endeffector; and a second proximal controller configured to generate asecond actuator feedback using the second actuator command and a stateof a second actuator coupled to the second distal joint through a secondtransmission element, and provide the second actuator feedback to thesecond actuator so that the second actuator causes correspondingmovement of the second distal joint and the end effector.
 24. Thecontrol system of claim 23, wherein the second transmission element ischaracterized by non-idealities.
 25. The control system according toclaim 23, wherein the end effector controller is configured to generatethe first distal joint command and the second distal joint command usingthe end effector command and a state of the end effector.
 26. A methodfor controlling movement of an end effector of a medical device,comprising: generating a first distal joint command using an endeffector command and a state of the end effector; generating a firstactuator command using the first distal joint command and a state of afirst distal joint, wherein the first distal joint is mechanicallycoupled to the end effector so as to be capable of moving the endeffector; generating a first actuator feedback using the first actuatorcommand and a state of a first actuator, wherein the first actuator ismechanically coupled to the first distal joint so as to be capable ofmoving the first distal joint; and providing the first actuator feedbackto the first actuator so as to cause corresponding movements of thestates of the first actuator, the first distal joint, and the endeffector.
 27. The method according to claim 26, further comprising:determining the state of the first distal joint by using informationprovided by a first distal joint sensor.
 28. The method according toclaim 26, further comprising: determining the state of the first distaljoint by using information generated from an image of the first distaljoint as provided by an image capturing device.
 29. The method accordingto claim 26, further comprising: determining the state of the endeffector by using information provided by an inertial sensor mounted onthe end effector.
 30. The method according to claim 26, furthercomprising: determining the state of the end effector by usinginformation provided by an accelerometer mounted on the end effector.31. The method according to claim 26, further comprising: determiningthe state of the end effector by using information generated from animage of the end effector as provided by an image capturing device. 32.The method according to claim 26, further comprising: compensating forfriction and compliance characteristics of one or more transmissionelements mechanically driving the first distal joint so as to ensuresteady state performance during the controlling of the movement of theend effector.
 33. The method according to claim 26, further comprising:compensating for friction and compliance characteristics of one or moretransmission elements mechanically driving the first distal joint so asto ensure accurate control of the movement of the end effector in arange of frequencies within a bandwidth of end effector commandsgeneratable by a surgeon.